Various metal ions are essential elements for the nutrition of plants. Deficiency of these metal ions in vegetation is often the cause of poor growth and yield of plants. Iron, manganese, zinc, copper, magnesium and calcium are all important in plant growth. Iron is particularly important.
One limiting factor in the absorption of iron, manganese, zinc and copper by plant roots is that their ions tend to form highly insoluble oxides and this limits their availability. For example, in soil iron ions are present either in their divalent (Fe.sup.+2) or trivalent (Fe.sup.+3) form with neutral or alkaline conditions favoring the latter. In the trivalent form, iron ions form highly stable precipitates having a solubility coefficient in the order of 10.sup.38. In consequence, soils in which there is a neutral or basic pH, such as calcareous soil in which the calcium carbonate acts as a buffer maintaining a high pH, iron precipitates rapidly and thus becomes unavailable to plant roots. Administration of iron in a divalent ionic salt has not proved to be sufficiently efficient. Administration of iron in the form of salts by means of spraying is common in agricultural practice. Preparations containing L-77 (Union Carbide), an expensive silicon based wetting agent, together with ferrous sulfate, have become a preparation of choice for various crops in which soil applied iron nutrients are neither sufficiently efficacious or cost effective. Swietlik D. and Faust M. 1984 (Horticultural Reviews, 6:287-356) recommend the use of such preparations for deciduous orchards. Examples are wax flower, persimmon, mango and citrus. Wax flower plants are members of the genus Chamaelaucium which belong to the Myrtaceae family.
Deficiency in iron causes plant leaves to loose chlorophyll. As the deficiency increases the leaves turn yellow or almost white. This symptom is called chlorosis.
Some plants such as wax flower need iron to be more freely available for absorption than other crops. Accordingly, for example, under Israeli environmental conditions it has become agricultural practice to supply iron by spraying the crops with iron sulfate mixed with L-77. During the warm season the crops are sprayed each week whilst in the winter spraying is less frequent.